1. Field of the Invention
The present invention relates to an image signal processing apparatus which processes image signals.
2. Related Background Art
Traditionally, as an apparatus to process image signals, there is an image signal coding apparatus which, in making digital transmission of image data obtained by digitalizing image signals, reduces the amount of data by compression-coding. One of the known coding methods applicable in the image signal coding apparatus is Differential Pulse Code Modulation (hereinafter referred to as DPCM), which compresses the amount of data by utilizing such characteristics as to create a close correlation between adjacent sample values in respect to samplings or image signals. This reduces the amount of pixel data, say into one-half, obtained by sampling and digitalizing the image signals. There is also a method to use compression-coding methods in combination. In this case, for the two kinds of color differential signals, respective sample data are reduced into one-half in the vertical direction of both the sides to make color differential line sequencing and further the sample data are halved in the horizontal direction of the screen to conduct DPCM.
FIG. 1 is a drawing showing a configuration of a conventional coding apparatus. The apparatus is designed to conduct the compression coding of color differential signals as mentioned above. FIG. 2 shows configuration of conventional decoding apparatus which corresponds to the coding apparatus shown in FIG. 1. FIGS. 3A to 3F are drawings to explain the operation of a conventional coding apparatus of FIG. 1 and a coding apparatus as one of the embodiments of the present invention which are explained later.
To start with, explanation is made about the operation of coding.
In FIG. 1, each of the two kinds of color differential data PR and PB, which have been sampled at specified sampling frequency, is input to respective input terminals 1 and 2. Then, by the operation of the switch 3 which can be switched over for each single horizontal scanning period by the horizontal synchronizing signal f.sub.s, the differential data are reduced into one-half in the vertical direction of the screen, subjected to the color differential line sequencing as shown in FIG. 3E and then supplied to the switch 4.
The switch 4 is turned on/off with the sampling offsets, as shown in FIG. 3F, the color differential data which have been line-sequenced with the above switch 3 frequency fs to reduce the data into one-half in the horizontal direction of the screen, which are then supplied to the subtracter 5. At subtracter, the predicted value data the preceding the value on output from the D flip-flop 9 is reduced from the input sample value data x.sub.i. The quantizer 6 quantizes the differential value data supplied from the subtracter 5 and outputs the DPCM data y.sub.i from the output terminal 10. The inverse quantizer 7 inversely quantizes the DPCM data y.sub.i output from the above quantizer 6 to output the differential representative value data. The adder 8 adds the predicted value to the preceding value data supplied from the above D flip-flop 9 to the differential representative value data output from the inverse quantizer 7 and then supplies it again to the D flip-flop 9. Then, the D flip-flop 9 delays the data supplied from the adder 8 by the amount corresponding to one sampling period and supplies them, as the predicted value data on the preceding data, to the subtracter 5 and the adder 8.
The operation of decoding is now explained.
To the input terminal 11 are fed, via a transmission line, the DPCM data which are output from the coding apparatus shown in the above FIG. 1. The inverse quantizer outputs the differential representative value data by inversely quantizing the DPCM data. The adder 13 adds and outputs the decoded value data on the preceding value supplied from the later-explained D flip-flop to the differential representative value data, which are output from said inverse quantizer. The data output from the adder 13 are delayed at the D flip-flop 14 by the amount corresponding to one sampling period and fed back to the above adder 13 as the decoded value data on the preceding value. The data output from the adder 13 are supplied to the interpolation circuit 15. In the interpolation circuit 15, each of the color differential data P.sub.R and P.sub.B, reduced to one-half in horizontal direction of the screen as shown in FIG. 3F, is subjected to interpolation processing and supplied to the switch 16. The switch 16 supplies the color differential data P.sub.R to the interpolation circuit 17 and the color differential data P.sub.B to the interpolation circuit 18 by switching alternately between the a terminal and the b terminal in synchronization with the horizontal synchronization signal f.sub.h. As shown in FIG. 3C, the interpolation circuit 17 outputs the color differential data P from the output terminal 19 as the color differential data P.sub.R shown in FIG. 3A by subjecting the data to the interpolation processing in the vertical direction of the screen. The interpolation circuit 18, as shown in FIG. 3D, outputs the color differential data P.sub.B from the output terminal 20 as the color differential data P.sub.B shown in FIG. 3B by subjecting them to the interpolation processing in the vertical direction of the screen.
However, in the above conventional example of coding apparatus and decoding apparatus, since the sample data are reduced to one-half in horizontal direction of the screen prior to the implementation of DPCM, the distance between each example data on the screen becomes 2/fs (fs means sampling frequency). Thus, the correlation between each sample data is less close, thereby causing a problem of large error of prediction.